Tyrosine Kinase Receptors (TKR) are known to be implicated in the initiation and progression of multiple multifunctional malignancies such as colorectal cancer (CRC), lung, head & neck, breast, prostate, gastrointestinal, brain tumors and melanoma.
Colorectal cancer is the second most common cause of cancer-related deaths in the developed world. In Europe, there has been an average annual estimate of 1.7 million cancer associated deaths (Ferlay J., Parkin D. M., Steliarova-Foucher E., Eur. J. Cancer, 2010 March:46(4):765-81).
The tyrosine kinase receptors (TKRs), largely epitomized by the ERB-B family of proto-oncogenes (ERB1-4), contain (i) an extracellular ligand-binding domain (domains I-IV), (ii) a single membrane spanning region, (iii) a juxtamembrane nuclear localization signal, and (iv) a cytoplasmic tyrosine kinase domain. ERBB1, the epidermal growth factor receptor-axis (EGFR-axis) has been the most comprehensively studied molecular target in oncology therapeutics over the past decade and is now a validated target for the treatment of cancer patients (Wheeler D. L., Dunn E. F., Harari P. M., Nat. Rev. Clin. Oncol., 2010 September; 7(9):493-507).
FDA approval for monoclonal antibodies (mAbs) against CRC has been active since 2004. In parallel, the rational design of quinazoline-based anti-EGFR tyrosine kinase inhibitors (TKIs) came to the fore. Unfortunately, coincident with this interest in targeting EGFR, was the identification of intrinsic and acquired resistance to EGFR inhibiting agents. The first uniform clinical definition of acquired resistance to EGFR inhibitors was reported in January 2010 (Jackman D., J. Clin. Oncol., 2010; 28:357-66).
The capacity of cancer cells to adapt to treatment suggests that additional mechanisms of resistance to EGFR inhibitors may have a key role in regulating tumor response, such as the induction of tumor/stromal interactions (angiogenesis), translocation of surface receptors to the nucleus, altered DNA damage response, and as yet undiscovered mutations. Intrinsic and acquired resistance to EGFR inhibitors is increasingly well-recognized, and epitomized by (i) genetic and epigenetic variations of genes in the EGFR oncogenic cascade (Yarden Y., Sliwkowski M. X., Nat. Rev. Mol. Cell. Biol., 2001 February; 2(2):127-37), (ii) expression of the EGFRvIII variant, deprived of the extracellular ligand binding domain (Learn C. A., Hartzell T. L., Wikstrand C. J., Archer G. E., Rich J. N., Friedman A. H., Friedman H. S., Bigner D. D., Sampson J. H., Clin. Cancer Res. 2004 May 1; 10(9):3216-24), and (iii) mitochondrial and nuclear translocation of constitutively activated EGFR (Lo H. W., Breast Cancer Res. Treat. (2006)95:211-18). As data accrues implicating the functional impact of EGFRvIII as well as mitochondrial and nuclear EGFR, it becomes increasingly important to understand the extent to which these mechanisms may contribute to the therapeutic response to EGFR-targeted therapies and design strategies for selective targeting of these variants.
In the post-genetics personalized medicine era, major issues challenging the successful application of targeted and personalized therapies along the TKR axes include: (i) early detection and intervention at the molecular level with concurrent probing of biomarkers underlying compromised responsiveness or resistance to targeted anti-TKR therapies; and (ii) management of resistance and reconstitution of responsiveness with minimally toxic therapies and strategies that enhance localized delivery. While several theranostic agents against TKR have been reported, they are based on monoclonal antibodies or anti-EGFR peptides, which explicitly target the extracellular binding domains thus sparing the truncated EGFRvIII variant.
Targeted therapies combined with imaging and/or sensing probes, also denoted as “beacons” comprise targeted theranostics. Near-infra-red (NIR) emitting probes are mostly preferred for biological applications due to: (a) lack of native NIR fluorescence in biological systems leading to highly sensitive detection, delineating molecular structures in pM (picomolar) concentrations (Weisseleder, 2003); (b) low scattering of NIR photons allowing for better image resolution (Cheong 1990); and (c) deep-in tissue penetration permitting non-invasive imaging (Sokolov 2003).
Magnetic resonance imaging (MRI), on the other hand, has high resolution but significant limitations due to inherent low sensitivity of the MR probes which makes validation of in vivo imaging experiments by more than one approach essential. This problem cannot be solved by merely adding two different classes of probes, e.g. optical and MRI together, unless they have identical pharmacokinetic properties (Flurrano, 2007).
Optical imaging with anti-EGFR antibodies failed to introduce a therapeutic functionality (EI-Sayed IH, 2005). Radiolabelled TKI, exhibit non-specific binding and suboptimal pharmacokinetic properties due to low hydrophilicity (Abourbech G, 2007). MRI probes are very limited (Suwa T, 1998). Thus, multimodal probes are needed in order to solve this problem and provide scaffolds for the amplification of MRI contrast agents.
The majority of bimodal agents reported so far include gadolinium complexes or iron oxide nanoparticles conjugated to fluorescent organic dyes (Mulder, 2007). Lanthanides, however, exhibit complementary properties over organic fluorophores, including resistance to photobleaching, long luminescence lifetimes, minimal or no reabsorption and sharp emission bands (Petoud, 2003; Zhang, 2005). The first successful attempt to develop lanthanide chelators combining both MRI and NIR luminescent activities has achieved long fluorescence lifetimes in metal-organic frameworks (Koullourou, 2008). Despite limited efforts to achieve bimodal imaging with mixtures of these agents (Manning, 2008), combining optical with magnetic properties within the same stable molecular structure also possessing anti-EGFR therapeutic potential, with optimized in vivo stability, targeting efficacy and desirable pharmacokinetics for clinical translation through high synthetic versatility remains a major challenge. Despite their favourable properties in term of sensing properties and drug delivery efficacies, the plethora of nanosized systems reported so far have not enjoyed regulatory approval (with the exception of a few examples in the polymeric and liposomal sub-group). Therefore, organometallic complexes with favourable pharmacokinetics like the complexes disclosed herein are of high promise for clinical use.
The past decades have witnessed well-documented epidemiologic associations of numerous natural products with cancer incidence and/or progression. Genomic and proteomic studies have correlated certain natural products, mainly isoprenoids, with molecular-level processes and have attributed to them functional relevance with specific molecular targets.
Natural triterpenic diols, extracted from olive oil, have been shown to promote apoptosis in astrocytoma cells through Reactive Oxygen Species (ROS)-mediated mitochondrial depolarization and Jun-Terminal Kinases (JNK) activation (Martin R., Ibeas E., Carvalho-Tavares J., Hernandez M., Ruiz-Gutierrez V., Nieto M. L., PLoS One. 2009 June 22; 4(6):e5975).
Natural terpenoids have also been implicated in the targeting of apoptosis pathways. Treatment of breast and prostate cancer with triterpenoids has been shown to target inflammatory pathways which can be exploited for the prevention and treatment of cancer (Yadav V. R., Prasad S., Sung B., Kannappan R., Aggarwal B. B., Toxins (Basel), 2010 October; 2(10):2428-66). Particular targets downstream of TKRs have also been identified as therapeutic targets of isoprenoids. Ras pathway activation in gliomas has proven to be a critical target of the treatment with intranasal administration of perillyl alcohol (da Fonseca C. O., Linden R., Futuro D., Gattass C. R., Quirico-Santos T., Arch. Immunol. Ther. Exp. (Warsz), 2008 July-August; 56(4):267-76). Further studies are currently trying to unravel the activities and propose a unifying mechanism of the anticancer actions of triterpenoids and their synthetic analogs (Safe S. H., Prather P. L., Brents L. K., Chadalapaka G., Jutooru I., Anticancer Agents Med. Chem., 2012 December; 12(10):1211-20).
Tocotrienols, a group of vitamin E derivatives, have been strongly associated with TKR pathways. Based on the inducible c-Rous sarcoma tyrosine kinase (c-SRC) inhibitory properties of tocotrienols, it may be postulated that tocotrienols have anti-cancer effects in SRC-mediated malignancies (e.g. gliomas, melanomas or malignancies associated with chronic inflammatory disorders). Antiproliferative effects of tocotrienols in pre-neoplastic mammary epithelial cells do not reflect a reduction in EGF-receptor mitogenic responsiveness. Rather, they result from the inhibition of early post-receptor events involved in cAMP production upstream from EGF-dependent MAPK and phosphoinositide 3-kinase/AKT mitogenic signalling (Sylvester P. W., Nachnani A., Shah S., Briski K. P., Asia Pac. J. Clin. Nutr., 2002; 11 Suppl 7:S452-9).
Recently, cancer stem cells, a cell population with dominant expression of TKR and a rapidly progressing area of cancer initiation and progression, have been implicated in the theory for cancer chemoprevention by natural dietary compounds (Li Y., Wicha M. S., Schwartz S. J., Sun D., J. Nutr. Biochem., 2011 September; 22(9):799-806). Retinoids, a nutrient category with strong epidemiologic association with cancer prevention, are shown to arrest breast cancer proliferation through selective reduction of the duration of receptor tyrosine kinase signalling (Tighe A. P., Talmage D. A., Exp. Cell. Res. 2004 December 10; 301(2):147-57). Despite these associations and multifaceted research, there has been no undertaking to exploit the signal modulating and anticancer attributes of selected phytochemicals within theranostic systems.